Hardware Buttons for USB / I2C Input

nice to be in this forum.
I am not really new to Arduino but the last project I did dates a few years back.
Right now I am building a hardware control for a raspberry pi running the OpenCPN navigation software.
In the final project there are going to be around 15 hardware buttons that will control various ascpects of the software.
My current idea is to somehow multiplex these buttons and then, using an Arduino type device send data to the Raspberry Pi either by USB or by I2C
For USB I was thinking of an Adafruit Trinket.
Can anyone here help me with the multiplexing of the hardware? I am not sure if there are enough pins on the arduino to achieve this.

Thanks for any suggestions!

Use a Nano, with USB to the Pi.
Don't need to bother with multiplexing, 18 IO are free to read buttons, D2 to D19, with D0/D1 in use for USB comm's.

Thank you. will go for it.
I should have mentioned that I'd need three other pins as well.
But I guess I can deal with that also.

I started to work with a Nano ( Nano Every ) and with my first Button to Keyboard test, I have gotten the Error:
"No Native USB support available on this board"
Is the Nano board not usable for this?

Nano (and Nano Every) don't support HID functionality (read: can't act as a keyboard). If you want to keep it small, Arduino Micro or Sparkfun ProMicro.

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